set request default format (http://stackoverflow.com/questions/4643738/rails-3-respond-to-default-format)

defort_format.rb

# first scheme
before_filter :default_format_xml

# Set format to xml unless client requires a specific format
# Works on Rails 3.0.9
def default_format_xml
  request.format = "xml" unless params[:format]
end

# second scheme
before_filter :default_format_json

def default_format_json
  if(request.headers["HTTP_ACCEPT"].nil? &&
     params[:format].nil?)
    request.format = "json"
  end
end